1. Field of Invention
The present invention relates to a surface light source device of side light type applied to backlighting, a liquid crystal display and a light guide plate suitable for use therein.
2. Related Art
Conventionally, a surface light source device of side light type is, for instance, applied to a backlighting arrangement of a liquid crystal display. This arrangement is suitable for making the overall shape of the device thinner.
Generally, the surface light source device of side light type uses a wedge-shaped light source, such as a cold cathode tube, as a primary light source which is provided at the side of a light guide plate (plate-like guide body). Illumination light emitted from the primary light source is fed into the guide plate through a side end face thereof. Having been fed inside, the illuminating light propagates through the guide plate, whereby light is output from a major surface of the guide plate toward a liquid crystal display.
Known types of guide plate include a guide plate having a generally uniform thickness and a guide plate which decreases in thickness as distance from the primary light source increases. Generally, the latter emits illuminating light output more effectively than the former.
FIG. 20 is an exploded perspective view of a surface light source device of side light type using the latter type of guide plate. FIG. 21 is a cross-sectional view taken along the line Axe2x80x94A of FIG. 20. As shown in FIG. 20 and FIG. 21, a surface light source device 1 comprises a guide plate 2, a primary light source 3, which comprises a wedge-shaped light source element 7 and a reflector 8, a reflection sheet 4 and light control members consisting of prism sheets 5 and 6.
The guide plate 2 comprises a scattering guide body, which is wedge-shaped in cross-section, known as a scattering guide plate. The scattering guide body is made of a material having a uniform scattering function, and comprises a matrix of, for instance, PMMA (polymethyl-methacrylate) and a great number of particles which are uniformly dispersed therein. The refractive index of these particles is different from that of the matrix.
The guide plate 2 has two major surfaces 2B and 2C. The major surface 2C is called an emission surface and outputs illuminating light. The other major surface is called a back surface.
The light source element 7 comprises, for instance, a cold cathode tube (fluorescent lamp), and a reflector 8, generally semicircular in cross-section, is provided to the rear of the light source element 7. Illumination light is supplied through the opening of the reflector 8 toward the side end surface of the guide plate 2. A sheet-like specular reflection member comprising metal foil or the like, or a sheet-like diffussive reflection member comprising white PET film or the like, is used as the reflection sheet 4.
After light L from the primary light source 3 has been led through the incidence surface 2A into the guide plate 2, inside the guide plate 2, the illuminating light L propagates toward the end while being repeatedly reflected between the emission surface 2C and the back surface 2B, along which the prism sheet 4 is arranged. During this time, the light receives the scattering action inside the guide plate 2. When a reflection sheet 4 having light-diffusion properties is used, dispersing action of this reflection sheet 4 also has effect. As a general tendency, the incidence angle to the emission surface 2C gradually decreases each time the light is reflected from the slope 2B. This increases components which are below the critical angle to the emission surface, facilitating emission from the emission surface. Consequently, insufficient emission of light in regions which are far from the primary light source 3 is prevented.
As a result of the above scattering action, the illuminating light emitted from the emission surface 2C has a great many propagation directions. However, the light is not completely scattered; the main direction of propagation inclines in the end direction (the opposite direction to the primary light source 3) with respect to the front direction. In other words, light emitted from the guide plate 2 has directivity. This property of the guide plate 2 is known as directional emission.
The prism sheets 5 and 6 are provided in order to correct this directivity. The prism sheets 5 and 6 comprise light-permeable sheet-like material such as, for instance, polycarbonate. In many cases, the prism surface of the prism sheet 5 is provided facing the scattering guide plate 2, while the prism surface of the prism sheet 6 is provided with its back facing the guide plate 2.
Each of the prism surfaces comprises a great number of micro-projections, triangular in cross-section, which run generally parallel to one direction. The inner side prism sheet 5 is provided so that its projections run generally parallel to the incidence surface 2A. The outer side prism sheet 6 is provided so that its projections run generally perpendicularly to the incidence surface 2A.
The slopes of these projections correct the main propagation direction of emitted light to the frontal direction of the emission surface 2C. Prism surfaces may provided on both faces; in other words, double-faced prism sheets may be used.
The above surface light source device of side light type generally emits light in the frontal direction more effectively than a device of the same type which uses a guide plate of practically uniform thickness.
However, the conventional device described above has a tendency whereby an undesirable region of low brightness is generated on the guide plate 2. This reduces the uniformity of the light output.
As shown for instance by reference mark AR1 in FIG. 20 and FIG. 22a, this low brightness region occurs in the vicinity of the incidence surface 2A.
Brightness distribution along the line Bxe2x80x94B in FIG. 22a varies depending on density of particles dispersed inside the scattering guide plate 2. FIG. 22b illustrates two examples of brightness distribution (high density/low density). As can be understood from this graph, it is difficult to make brightness distribution flat, and thereby eliminate the low brightness region AR1, merely by adjusting the particle density.
As shown in FIG. 22a, the low brightness region AR1 tends to spread near the corners. The positions of the corners correspond to electrodes 7A and 7B of a wedge-shaped lamp 7. The low brightness region AR1 is liable to spread here since the electrodes 7A and 7B have weaker light supply power than the center 7C.
The following two solutions (1) and (2) to this problem have been proposed.
(1) According to a first proposed solution, as shown in the cross-section in FIG. 22c, the back surface 2B of the scattering guide plate 2 is shaped in a gentle curve, and particle density is adjusted so that brightness distribution becomes flat.
(2) According to another proposed solution, as shown in FIG. 23, groups of surface region elements 9, which have scattering a function; are provided in patterns of pitch P on the back surface 2B of the scattering guide plate 2. These are known as scattering patterns. The scattering patterns 9 increase the amount of illuminating light components below the critical angle to the emission surface 2C in the vicinity of the scattering patterns 9, consequently facilitating light emission.
The covering rate (occupancy rate) of the scattering patterns 9 increases near the edges. The scattering patterns 9 can be formed by local deposition of scattering ink, or by locally making the back surface 2B rough.
These proposed solutions are easily implemented when the back surface of the guide plate is basically a flat surface, but are difficult to implement when a great number of projections are provided on the back surface of the guide plate.
FIG. 24 is an exploded perspective view of a conventional surface light source device of side light type 10 which uses such a scattering guide plate 12. The surface light source device 10 is the same as the surface light source device 1 shown in FIG. 20, except that prism sheet 6 is not used and structure of the guide plate 12 is different. Therefore, explanation of like parts will not be repeated.
The guide plate 12 comprises the same scattering guide body as the guide plate 2 (see FIG. 20), but has a light control surface prism surface) provided on the back surface 12B of the guide plate 12. The light control surface has a great number of prism-like projections provided in rows. These projections are provided repeatedly and generally perpendicular to the incidence surface 12A. Slopes of the projections correct directivity of illuminating light emitted from the emission surface 12C. Correction is performed by shifting the preferential propagation direction closer to a frontal direction within a surface parallel to the incidence surface 12A.
The direction in which the rows of projections run is generally perpendicular to the prism sheet 5 (shown in partial enlargement at reference marks C and D). That is, the back surface (light control surface) 12B fulfils generally the same function of correcting directivity as the prism sheet 6. Furthermore, since one of the prism sheets is removed, not only can we expect a reduction of light loss, but also the overall structure is simplified.
However, as shown in the example of FIG. 24, in a structure using a scattering guide plate 12 which has such a light control surface, an undesirable low brightness region AR1 tends to be generated in the vicinity of the incidence surface 12A and particularly around the corners. Indeed, this tendency is even stronger than in structure (without a light control surface on the back surface of the guide plate) shown in FIG. 20. This is probably because the light control surface restricts light from spreading in the left and right directions as viewed from the incidence surface 12A.
In practice, application of the above proposed solutions (1) and (2) leads to difficulties. Design becomes extremely complex when attempting to curve the back surface 12B, which has a great number of projections, in compliance with first proposed solution, and in practice it is difficult to achieve uniform brightness distribution.
On the other hand, when attempting to realize uniform brightness distribution by providing scattering patterns in compliance with the second proposed solution (2), scattering patterns 9 of extremely large area are needed. As a result, the scattering pattern 9 becomes visible above the emission surface. Obviously, this causes a noticeable reduction in illumination output.
It is to be noted that, even when a light control surface is provided on the emission surface, a low brightness region of generally the same manner emerges. In this case, it is also difficult to obtain satisfactory results when the proposed solutions (1) and (2) are applied in practice.
The present invention has been realized after consideration of the above background. Accordingly, it is an object of the present invention to improve a surface light source device of side light type wherein a scattering pattern can be applied without loss of illumination output quality, even when a light control surface is provided on a major surface (back surface or emission surface) of a guide plate.
It is another object of the present invention to provide a liquid crystal display in which such an improved surface light source device is applied as backlighting. It is yet another object of the present invention to provide a guide plate which is suitable for the above improvements.
The present invention provides a new guide plate. This guide plate is incorporated in a surface light source device of side light type, or in a liquid crystal display which is backlighted by the same.
The guide plate has an emission surface and a back surface as its major surfaces, light being fed in from an end surface, bending and finally being emitted from the emission surface. A light control surface, comprising a great number of projections running generally perpendicular to the incidence surface, is provided on the emission surface or the back surface, and a scattering pattern, comprising a great number of hardly visible micro-dots having a scattering function, is provided on the emission surface or the back surface.
In a typical case, the covering rate of the scattering pattern increases in accordance with the distance from the incidence surface, and, decreases as the distance from the corners increases. This tendency of covering rate corrects brightness distribution on the emission surface and prevents insufficient brightness, which is especially liable to occur near the corners.
Actual size of the micro-dots is not greater than 80 xcexcm. More preferably, not greater than 50 xcexcm. The micro-dots may comprise a locally rough surface provided on the emission surface or the back surface.
The micro-dot arrangement is random as long as it complies with the typical manner of arranging the micro-dots. The degree of randomness may also differ according to direction.
When the object to be illuminated is a color liquid crystal display, the degree of randomness should preferably differ according to direction. Consequently, moarxc3xa9 streaks are prevented. The micro-dots may be provided in a locally random arrangement so that they do not overlap with a specific color filter of the liquid crystal panel, which is the illuminated object.
Next, the present invention will be explained in detail with reference to the accompanying drawings.